This invention relates to switched reluctance (SRM) motors and, more particularly, to a method of reducing motor noise by controlling or manipulating a current profile for a motor.
Switched reluctance, or SRM motors are well-known in the art. One problem with operating these motors is noise. As discussed in my related U.S. Pat. No. 5,446,359, one source of this noise is caused by the dissipation of current in a motor's phase windings as each phase is switched at the end of its cycle. As the motor is switched from one phase to another, the energy, which represents approximately thirty percent (30%) of the energy supplied to the phase winding during its active period, decays off. As described in the application, a portion of this energy is either recovered using a storage capacitor, or dissipated.
In general, it will be understood that a major cause of motor noise is an abrupt change in the normal forces to which the motor is subjected. The noise is particularly acute if any of these transitions occur when at the maximum deflection points in the a motor cycle. In coassigned U.S. Pat. No. 4,239,217, there is described a phenomenon referred to as "ovalizing". This is a condition in which the general circular shaped rotor and stator of a motor tend to be distorted into an oval shape by the forces produced when rotor poles and stator poles come into alignment. These earlier applications further describe techniques by which these ovalizing forces can be minimized.
It will be understood that while the approaches set forth in the earlier applications are effective in reducing motor noise, other efforts to further reduce motor noise are also important. One of these is to control the current profile of each motor phase. One control scheme by which current is controlled to minimize torque, uses various look-up tables or schedules to effect control. (See U.S. Pat. No. 4,868,477 to Anderson et al.) It is also known to control a current profile by advancing the length of a phase thereby increasing motor dwell time for the phase. (See U.S. Pat. No. 4,253,053 to Ray et al.) A third scheme is discussed by S. Chan and H. R. Bolton in their paper Performance Enhancement of Single-phase Switched Reluctance Motor by DC Link Voltage Boosting; IEEE Proceedings-B, Volume 140, Paper No. 5, September, 1993. Here, current is put into a phase and subsequently taken out at a faster than normal rate by employing a boost voltage at the beginning and end of a phase. A drawback with this approach is its inflexibility.
While the above approaches may be effective, they require a significant amount of circuitry, or a significant amount of manipulation, to achieve a reasonable amount of control. Other ways of current profile control may be more effective and easier to achieve. It is important to understand that noise reduction is accomplished by matching as closely as possible the slope of the current profile at the end of the active portion of a phase with the slope of the tail current decay portion of the profile. There is disclosed apparatus and a method for controlling tail current decay in related U.S. Pat. No. 5,446,359. In accordance with the disclosure therein, the slope of the decay portion of the profile is controllable. By using the circuitry and method as disclosed herein, the phase active portion of the profile is controllable so a smoother transition is now obtainable, without effecting the period of the phase or other significant motor operating parameters to effect significant noise reduction.
During any phase of motor operation, there are essentially three intervals. First, is a ramp-up from a zero current level to a peak or near-peak current level. Second, is an interval in which the current is rounded or profiled. Third, is the current decay or tail current interval. Tail current can be defined as a downward trend in current (from a positive current level toward zero current) when the current level would otherwise be expected to be generally flat. Throughout the entire phase interval (zero level to peak, and back to zero) the current supplied to a motor phase winding is pulse width modulated. From approximately the peak current level to current cut-off, and from cut-off to zero current, the profiling is achieved by using hard chopping or soft chopping, or both, of the input current waveform. As noted above, under present current control schemes, there is a transition point which occurs at cut-off. This cut-off point defines the beginning of the current recovery period since current can be extracted from the phase winding by dissipation in the winding, or by recovering a portion of the current into an energy storage device such as a capacitor. In prior attempts to control tail current, there has been an effort to modify the abrupt transition which occurs at cut-off. This is done in an effort to reduce the normal formals produced in the motor at this time and, in turn, reduce the undesirable motor noise. However, the transitional change is still so significantly pronounced that there is still excessive ringing which, if eliminated, will enhance motor performance.
In related U.S. Pat. No. 5,446,359, tail current control was achieved by controlling the rate of decay using pulse width modulation (PWM). This involved zero volt chopping, using PW or control signals, to produce a desired slope in the current decay portion of the profile. In co-pending application Ser. No. 08/187,532 of which this is a continuation-in-part, the slope of the current profile was controlled in a portion of the active region of the waveform. This was done using hard or soft chopping techniques and was done to lessen the transition which occurs in the slope at the transition between the active and inactive (or tail current decay) portions of the waveform. Again, one objective of this is to reduce the ringing or noise which results from an abrupt transition to produce a smoother running motor and one subjected to reduced internal motor forces.